FEB 1, 2010 - Planet Definition Doesn't Apply Beyond Solar System revisits the silliness visited upon us by the International Astronomical Union (IAU) back in 2006, when they demoted Pluto from planet to dwarf planet. A problem astronomers are running up against, now that they've discovered hundreds of planets orbiting faraway stars (exoplanets), is that the definition of a planet adopted by the Pluto-bashers may not fit these planets being discovered in other stellar systems. Presumably these objects are roughly spherical, which is part of the IAU definition. But have they cleared their orbits, as required by the IAU to qualify as a planet? Many of these bodies share characteristics with Pluto, such as a highly elliptical and steeply inclined orbit. Science writer Ray Villard raises these questions and more about what it means to be a planet, and notes in passing that it's a definition "which goes back to antiquity." Indeed, it was astrologers who defined planets a few thousand years ago. And we defined them as bodies moving in regular, repeated orbits against the backdrop of the faraway stars - which makes Pluto a planet; and the exoplanets planets too, if you happen to be viewing them from another planet within their local system.
